Ultrasonic transducers have been used in a number of sensing applications such as medical imaging, non-destructive evaluation, gas metering and a number of ultrasound generating application such medical therapy, industrial cleaning, etc. One class of such transducers is the electrostatic transducers. Electrostatic transducers have long been used for receiving and generating acoustic waves. Large area electrostatic transducer arrays have been use for acoustic imaging. The electrostatic transducers employ resilient membranes with very little inertia substrate which forms the second electrode. When distances between the electrodes are small the transducers can exert very large forces against a fluid in contact with the membrane. The momentum carried by approximately half a wavelength of air molecules in contact with the upper surface is able to set the membrane in motion and vice versa. Electrostatic actuation and detection enables the realization and control of such membranes.
Broad band microfabricated capacitive ultrasonic transducers (cMUTs) may include multiple elements each including membranes of identical or different sizes and shapes supported above a silicon substrate by walls of an insulating material which together with the membrane and substrate define cells. The walls are formed by micromachining a layer of insulation material such as silicon oxide, silicon nitride, etc. The substrate can be glass or other substrate material. The capacitive transducer is formed by a conductive layer on the membrane and conductive means such as a layer either applied to the substrate or the substrate having conductive regions. A single cell of a cMUT is illustrated in FIG. 1. The cMUT includes a bottom electrode 11 and a top electrode or membrane 12 supported by insulating walls 13. When suitable AC and DC voltages are applied between the electrodes electrostatic forces cause the membrane to oscillate and generate acoustic waves. Alternately a DC voltage applied between the electrodes can be modulated by oscillation of the membrane resulting from sound waves stricking the membrane. The cMUT includes an isolation layer 14 such as an oxide layer to prevent shorting between the electrodes if the membrane is deflected into contact the bottom wall of the cell 16.
The electric field between the electrodes can attract and trap charges 17 either on the surface of or in the isolation layer 14. The charges stay in the trapping cites for a long period because there is no DC path to discharge them. The accumulated charge shifts the DC voltage between the two electrodes away from the applied voltage by a random value. This dramatically degrades the reliability and repeatability of device performance.